1. Field of the Invention
This invention relates to an external cavity type semiconductor laser apparatus in which light from the light-emitting rear facet of a semiconductor laser device is reflected by an external reflector and returns to the said semiconductor laser device.
2. Description of the Prior Art
Conventional semiconductor laser devices have been designed so that the oscillation longitudinal mode of the laser devices can be selected depending upon the gain distribution of laser media and the transmission characteristics of the laser resonator. FIGS. 6(a) to 6(c) show the selectivity of the oscillation longitudinal mode of a conventional semiconductor laser device, wherein FIG. 6(a) shows the relationship between the wavelength and the gain distribution of laser media, FIG. 6(b) shows the relationship between the wavelength and the spectrum of each longitudinal mode, and FIG. 6(c) shows the spectrum in the superradiant situation that is obtained by the superposition of the characteristic curves of FIGS. 6(a) and 6(b). In general, of the longitudinal modes of laser light produced by a semiconductor laser device, a longitudinal mode of laser light with a wavelength that is the closest to the peak (the maximum value) of a gain distribution receives the greatest gain and becomes an oscillation longitudinal mode. However, the bandgap of the semiconductor materials of the laser device varies with changes in ambient temperature, and accordingly the wavelength peak of the gain distribution is shifted to the longer wavelength side at the rate of 2-3 .ANG./deg. Moreover, not only the refractive index of laser media varies, but the laser cavity length varies with changes in the thermal expansion of the laser device, so that the said longitudinal modes are shifted to the longer wavelength side at the rate of about 0.7 .ANG./deg., while maintaining a longitudinal mode spacing of about 3 .ANG. therebetween. Thus, when ambient temperature rises, since the amount of change in the gain distribution is greater than that of change in the longitudinal modes at the beginning, the oscillation wavelength continuously varies over a period of time. When the temperature further rises, the oscillation wavelength brings about mode hopping, after which the oscillation wavelength causes the above process to repeat itself resulting in continuous changes and further mode hopping, as shown in FIG. 8, thereby attaining a step function type change. Moreover, the wavelength also varies with changes in current for driving the laser device. The above-mentioned phenomena prevent the application of conventional semiconductor laser devices as a light source for either wavelength multiplex optical-communication or a high resolution spectrometer.
In order to solve these problems, an SEC laser apparatus (short external cavity laser diode) has been invented in which laser light from the light-emitting rear facet of a semiconductor laser device is reflected by an external reflector and returns to the said semiconductor laser device. The oscillation longitudinal mode of the said laser device of the SEC laser apparatus is selected depending upon three factors, namely, the gain distribution of the laser, the longitudinal modes, and the selectivity of wavelength of the external cavity, which are shown in FIGS. 7(a), 7(b) and 7(d) in contradistinction to those of FIGS. 6(a) to 6(c). FIG. 7(a) shows the relationship between the wavelength and the gain distribution of laser media, FIG. 7(b) shows the relationship between the wavelength and the spectrum of each longitudinal mode, FIG. 7(c) shows the relationship between the wavelength and the resonance characteristics of the external cavity, and FIG. 7(d) shows the spectrum in the superradiant situation that is obtained by the superposition of the characteristic curves of FIGS. 7(a) to 7(c). The envelope curve of the spectrum in the superradiant situation of FIG. 7(d) is of a ripple, whereas that of the spectrum in the superradiant situation of FIG. 6(c) is of a smooth circular arch. The temperature characteristics of the peak of the envelope curve can be controlled by changes in the external cavity length (i.e., the distance between the semiconductor laser device and the external reflector), thereby attaining the suppression of mode hopping.
The characteristics of an oscillation wavelength with respect to temperatures of an ordinary SEC laser apparatus are shown in FIGS. 9(a) to 9(c), indicating that the same longitudinal mode is maintained in a temperature range of .DELTA.t; that a longitudinal mode successively receives the greatest gain in the same peak of the envelope curve of the spectrum shown in FIG. 7(d) in a temperature range of .DELTA.T and becomes an oscillation longitudinal mode; and that when the temperature range exceeds .DELTA.T, the oscillation longitudinal mode is shifted to the adjacent peak of the envelope curve, resulting in significant and sudden change of the oscillation longitudinal wavelength (i.e., mode hopping).
Moreover, FIG. 9(a) indicates that the temperature coefficient of the wavelength of the peak of the envelope curve shown in FIG. 7(d), d.lambda./dT, and the temperature coefficient of the longitudinal modes shown in FIG. 7(b), .gamma., have the following relationships: when d.lambda./dT&lt;.gamma., the oscillation longitudinal mode is successively shifted to a longitudinal mode that is positioned at the short wavelength side and a small mode hopping in a temperature range of .DELTA.T results. FIG. 9(b) indicates that when d.lambda./dT=.gamma., the value of .DELTA.T becomes equal to .DELTA.t, and the mode hopping does not occur in the range of .DELTA.T. FIG. 9(c) indicates that when d.lambda./dT&gt;.gamma., the oscillation longitudinal mode is successively shifted to a longitudinal mode that is adjacent to the long wavelength side, resulting in a small mode hopping in the range of .DELTA.T.
The above-mentioned SEC laser apparatus is composed of a VSIS (V-channelled Substrate Inner Stripe) double-heterostructure semiconductor laser device with an active layer of, for example, AlGaAs grown on a GaAs substrate, and an external reflector made of a GaAs chip the cleaved facets of which are coated with dielectric films so as to have a high reflectivity. Both the semiconductor laser device and the external reflector are mounted on a single mounting base of Cu with a space (i.e., the external cavity length) between the light-emitting rear facet of the said laser device and the reflecting face of the said external reflector. The reason why Cu is used for the mounting base is that the heat conductivity of Cu is so great that the semiconductor laser device can achieve an excellent heat-radiation. However, Cu has a great thermal expansion coefficient.
With an SEC laser apparatus that is composed of a semiconductor laser device with a length of 250 .mu.m and an external reflector with a length of 250 .mu.m, which are mounted on such a Cu mounting base, when the length of the external cavity is 60 .mu.m, it is possible that d.lambda./dT (i.e., the temperature coefficient of the wavelength of the peak of the envelope curve shown in FIG. 7(d)) agrees with the temperature coefficient (0.7 .ANG./.degree.C.) of the longitudinal mode of laser, and accordingly, as shown in FIG. 9(b), no mode-hopping occurs in the range of .DELTA.T and a specific lasing mode can be maintained, wherein .DELTA.T is 28.degree. C.
However, it is difficult in practice to control the temperature (at both ends of arrow mark shown in FIG. 9(b)) at which great mode hopping occurs. Accordingly, in the case where a specific temperature range in which mode hopping does not occur must be selected, the yield of laser devices decreases and/or a large amount of time is needed for the selection of characteristics. In order to solve these problems, it is necessary to enlarge the temperature range .DELTA.T in which mode hopping does not occur and to heighten the probability that a specific temperature range falls within the value of .DELTA.T.